1995 Chevy Corsica Trying to figure out why my brakes are d

I've had my car to a mechanic and my brakes still do this. They replaced the abs sensor. This is very hard to explain but I'm going to explain it the best way I can. My car doesn't do this all the time, but it mainly happens when I'm pulling out of a parking lot, drive way, etc. It only happens when I am going at a real low speed, and maybe when I have the steering wheel turned a certain way, I'll put on the brakes & they don't work and it makes this low tone noise in the car that I really can't describe, What could be causing the brakes not to work?